Electrodeposition (also known as e-coat) and autodeposition are different methods of depositing an organic polymer-containing coating on metallic surfaces. The two methods are different in several aspects.
Autodeposition has been in commercial use on steel for about thirty years and is now well established for that use. For details, see for example, U.S. Pat. Nos. 3,063,877; 3,585,084; 3,592,699; 3,674,567; 3,791,431; 3,795,546; 4,030,945; 4,108,817; 4,178,400; 4,186,226; 4,242,379; 4,234,704; 4,636,264; 4,636,265; 4,800,106; and 5,342,694. Epoxy polymer-based autodeposition coating systems are described in U.S. Pat. No. 4,180,603 (Howell. Jr.); U.S. Pat. No. 4,289,826 (Howell Jr.); U.S. Pat. No. 5,500,460 (Ahmed et al.); and International Publication Number WO 00/71337. The disclosures of all these patents and published patent applications are hereby incorporated by reference to the extent that they are not specifically contradicted by the below teachings.
Autodeposition compositions are usually in the form of a liquid, usually aqueous solutions, emulsions or dispersions in which active metal surfaces of inserted objects are coated with an adherent polymer or polymer film that increases in thickness the longer the metal remains in the bath, even though the liquid is stable for a long time against spontaneous precipitation or flocculation of any polymer or polymer, in the absence of contact with the active metal. When used in the autodeposition process, the composition when cured forms a polymeric coating. “Active metal” is defined as metal that spontaneously begins to dissolve at a substantial rate when introduced into the liquid solution or dispersion. Such compositions, and processes of forming a coating on a metal surface using such compositions, are commonly denoted in the art, and in this specification, as “autodeposition” or “autodepositing” compositions, dispersions, emulsions, suspensions, baths, solutions, processes, methods or a like term. Autodeposition is often contrasted with electrodeposition. Although each can produce adherent films with similar performance characteristics, the dispersions from which they are produced and the mechanism by which they deposit are distinctly different.
E-coat is a dispersion of organic polymers and de-ionized water, which is in a generally stably dispersed state and does not deposit coatings in the absence of electrical current. The e-coat dispersion may also comprise solvent and some ionic components. When a D.C. voltage is applied across two electrodes immersed in the e-coat dispersion, the passage of current is accompanied by electrolysis of water. This results in oxygen gas being liberated at the anode (positive electrode) and hydrogen gas liberated at the cathode (negative electrode). The liberation of these gases disturbs the hydrogen ion equilibrium in the water immediately surrounding the electrodes. This results in a corresponding pH change and this in turn de-stabilizes the paint components of the dispersion and they coagulate onto the appropriate electrode.
Typical e-coat belongs to two types: anodic and cathodic electrodeposition.
Both anodic and cathodic electrodeposition processes and coatings have the following disadvantages: Limited throwing power, that is thinner or no coating of substrates' inner surfaces due to uneven distribution of the electrical field; the requirement for a pretreatment coating deposited on metal surfaces prior to electrodeposition paint coatings to improve corrosion resistance; poor heat resistance of resulting coatings, typically electrodeposition coatings are not resistant to temperatures greater than 200 degrees C.; poor flexibility of the coating leading to inconsistent or unacceptable performance in Reverse Impact ASTM D5420 testing. Commercially useful voltages for satisfactory electrodeposition in automotive industries commonly range from 200-400 volts making the process expensive to run. Both anodic and cathodic electrodeposition processes are multi-step processes requiring time and manufacturing floor space fore each step.
A typical E-coat Process Sequence includes:                1. Spray degreasing        2. Immersion degreasing        3. Water rinse.        4. Surface activation        5. Zinc phosphate coating (chromium free).        6. Demineralized water rinse dip        7. Demineralized water rinse dip        8. Demineralized water rinse spray        9. E-coating coating thickness from 10 up to 40 μm        10. UF cascade spray rinse.        11. UF rinse.        12. Demineralized water rinse spray        13. Drying of the parts—compressed air blow.        14. Three stage electric or gas baking of coating with thermal, infrared stove drying oven.        
In anodic electrodeposition, a negatively charged organic polymer is dispersed in deionized water. Most anodic electrodeposition coating are derived from low molecular weight (1000 to 10,000 Daltons) solution polymerized, polymers having bound carboxylic acid groups to make them self dispersible. A substrate is submerged in an aqueous bath containing the negatively charged organic polymer and electricity is run through the bath with the substrate as the anode (positive electrode of the electrolytic cell). The negatively charged organic polymer moves to the anode and neutralization at the anode surface with the H+ ions generated from electrolysis water causes deposition of the negatively charged organic polymer on the anode.
In cathodic electrodeposition, a positively charged organic polymer is dispersed in deionized water. A substrate is submerged in an aqueous bath containing the positively charged organic polymer and electricity is run through the bath with the substrate as the cathode (negative electrode of the electrolytic cell). The positively charged organic polymer moves to the cathode and neutralization at the cathode surface with the OH-(hydroxide) ions generated at the cathode as result of electrolysis of water causes deposition of the positively charged organic polymer on the cathode.
Electrodeposition processing generally requires a pretreatment because the wet film is insufficiently porous to allow penetration of post treatments through the paint film to the substrate. Treatment agents which might desirably be introduced through the paint film to the underlying substrate cannot be utilized. In contrast, the initial wet film of an uncured autodeposited coating has sufficient porosity to allow use of treatment agents which penetrate the uncured autodeposition coating reach the underlying substrate.
Autodeposition compositions include organic polymers, often with surfactant molecules adsorbed on the polymeric species, dispersed in an aqueous liquid. Working autodeposition baths include significant amounts of activators for driving the autodeposition reaction, but are generally stable against coagulation or reaction in the absence of an active metal. In autodeposition processes, an active metal substrate is contacted with the autodeposition bath and cations are generated by dissolution of metals from the substrate by activators present in autodeposition baths without the need for applied electrical current. These cations complex with dispersed organic polymer species at the interface between the metal substrate and the autodeposition bath resulting in deposition of the organic polymer species and metals from the substrate onto surfaces of the metal substrate.
Conventional autodeposition treatment does not utilize an applied electrical current for deposition of a coating. In a typical autodeposition bath, generation of cations depends on the chemical dissolution of metals and follows galvanic series. This difference in dissolution creates galvanic couple formation at edges and defected areas where two metals are in contact with each other. The less active metal acts as a cathode and does not generate any cationic species to react with polymer particles resulting in nondeposition of polymer onto cathodic areas, i.e. edges and exposed noble areas of the galvanic couple, for example a cut edge of a galvanized substrate. These reactions lead to difficulty in uniformly coating composite metal parts or assemblies made up of different metal substrates, such as steel surfaces, zinc surfaces and aluminum surfaces. The different metals that make up the parts or assemblies each have different activity in the autodeposition bath relative to each other, which results in different coating characteristics.
It is desirable to develop a coating process that overcomes at least some of these drawbacks. Applicants have developed a coating bath and process utilizing a small applied potential to initiate and drive deposition in conjunction with etchant. This process overcomes the galvanic effect and allows the polymers to deposit evenly on dissimilar metals of an article to be coated.